Gradual genome stabilisation by progressive reduction of the genome in an interspecific hybrid with

Abstract

Considerable amounts of molecular and genetic data indicate that interspecific hybridisation may not be rare among natural strains of Saccharomyces sensu stricto. Although a post-zygotic barrier operating during meiosis usually prevents the production of viable spores, stable hybrids can arise which can even evolve into distinct species. This study was aimed to analyse the genome of a fertile Saccharomyces cerevisiae x S. uvarum hybrid and monitor its changes over four filial generations of viable spores. The molecular genetic analysis demonstrated that the two species did not contribute equally to the formation and stabilisation of the hybrid genome. S. cerevisiae provided the mitochondrial DNA and the more stable part of the nuclear genome. The S. uvarum part of the hybrid nuclear genome became progressively smaller by loosing complete chromosomes and genetic markers in the course of successive meiotic divisions. Certain S. uvarum chromosomes were eliminated and/or underwent rearrangements in interactions with S. cerevisiae chromosomes. Numerous S. uvarum chromosomes acquired S. cerevisiae telomere sequences. The gradual elimination of large parts of the S. uvarum genome was associated with a progressive increase of sporulation efficiency. We hypothesise that this sort of genomic alterations may contribute to speciation in Saccharomyces sensu stricto.

Evolutionary Role of Interspecies Hybridization and Genetic Exchanges in Yeasts

“…First, a gradual recovery of meiotic fertility follows genome stabilization of hybrid mitotic lines. This was demonstrated in an experimental study of an S. cerevisiae-S. uvarum hybrid line over four filial generations (6). During the successive generations, a gradual reduction of the S. uvarum genome occurred amid rearrangements with the S. cerevisiae genome, and a progressive increase of sporulation efficiency and ascospore viability was observed.…”

“…The classical genetic and molecular methods applied, such as sporulation and crosses, karyotyping, AFLP analysis, DNA-DNA hybridization, and the partial sequencing of a few selected genes, were not sufficient to fully characterize the hybrid genomes. These methods were sufficient, however, to indicate that hybrid lines generally undergo progressive genome stabilization, during which large genomic rearrangements occur, including aneuploidization, chromosomal translocation, and partial or total chromosome loss (6,42,90,99,123,174,175,198). Gene amplification also occurs, especially in subtelomeric regions of chromosomes.…”

“…In reality, this degree is measured from the average for many tetrads. (1) Gradual recovery of fertility can be obtained by successive mating/sporulation cycles, corresponding to gradual genome stabilization of the hybrids (6). (2) Hybrid speciation can be obtained by mating-type switching in the haploid clones issued from live ascospores, followed by the formation of homozygous diploids (71).…”

Polyploidy in fungi: evolution after whole-genome duplication

“…Hybridization may be followed by backcrosses with one parental species, allowing the recovering of a parental-like species bearing a few introgressed genomic parts as in the wet rot fungus Coniophora puteana [65] or other Saccharomyces species [66,67], and sometimes uncovering the sterility associated with interspecific hybridization [68]. Finally, in the most extremes cases, hybridization may lead to hybrid speciation (also called homoploid speciation) as in many plant and animal taxa [69,70].…”

Lager Yeast Comes of Age

“…This information was directly used to study the gene-based phenotypes, e.g., of sugar fermentations in yeast hybrids (108). The ease with which Saccharomyces strains can form viable hybrids results in hybrid formation also in natural environments (109)(110)(111)(112)(113). Based on the lack of apparent (or effective) prezygotic barriers, it has been proposed that speciation in Saccharomyces is ensured by postzygotic barriers preventing sporulation or the generation of viable spores.…”